Grease and line two 20cm (8") round cake tins with silicone-coated parchment paper, or line with greaseproof paper and grease with sunflower oil.
Preheat oven to 180°c.
Combine the gluten-free flour, maize flour, ground almonds, baking powder, mixed spice and salt in a bowl.
Sieve together four times to make sure the baking powder is evenly distributed
Add the grated carrot and raisins to the flour and stir together.
Measure the sunflower oil, eggs, brown sugar and vanilla extract into a large mixing bowl. Whisk together until pale in colour then stir in the dry ingredients.
Pour into the prepared cake tins and bake for 40 to 50 minutes until a knife inserted comes out clean.
Leave to cool in the tin for 15 minutes before turning out onto a plate. Now turn this onto the cooling wire so the cake cools the right way up or it could break in half.
For the topping, beat the butter to soften it using a food mixer or a hand-held electric mixer.
Add the icing sugar and mix into the butter slowly.
Stir in the finely grated orange zest to taste.
Once the icing sugar is mixed into the butter, increase the speed and beat until light and fluffy. Add a few drops of water if the buttercream is too firm.